Why can't I puppet a City-State in Civ V?

I started a game of Civilisation V with God & Kings expansion (GOTY edition, so a bunch of DLC as well). Started as Genghis Khan and warred a bit for prospective expansion areas, gaining an enemy capital, that I happily annexed. Then I turned my eye towards neighbouring city-states aiming to claim their luxury resources by conquering and puppeting them, but to no avail. Game didn't prompt me for choice, as I entered the city with my infantry, it immediately became annexed. Now I cannot raze or puppet the captured city-state. Why? Is this a game bug?

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How do you puppet a City-State in Civ 5?

Any puppet city can be annexed anytime by clicking on the city's banner (unless you're playing as the Venetians), and the city name can be changed while viewing the city.

Can you raze a City-State?

You can raze city states forever. If you choose to capture them they will not be occupied and will behave as a normal city. Liberation is a third option if you are recapturing a city state from another player.

How do you annex a City-State?

If you are a Suzerain of a City-State and you have 15 or more Envoys in it, you can directly annex it via a City-State diplomacy button.

Liz Phair - Why Can't I? (Official Video)

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Images: Geometric Photography, Vlada Karpovich, Kampus Production, Anna Shvets